linux-ext4 - Re: [PATCH 2/5] vfs: create a generic checking function for FS_IOC

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Date:   Wed, 26 Jun 2019 08:35:42 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     matthew.garrett@...ula.com, yuchao0@...wei.com, tytso@....edu,
        shaggy@...nel.org, ard.biesheuvel@...aro.org, josef@...icpanda.com,
        hch@...radead.org, clm@...com, adilger.kernel@...ger.ca,
        jk@...abs.org, jack@...e.com, dsterba@...e.com, jaegeuk@...nel.org,
        cluster-devel@...hat.com, jfs-discussion@...ts.sourceforge.net,
        linux-efi@...r.kernel.org, Jan Kara <jack@...e.cz>,
        reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        linux-nilfs@...r.kernel.org, linux-mtd@...ts.infradead.org,
        ocfs2-devel@....oracle.com, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 2/5] vfs: create a generic checking function for
 FS_IOC_FSSETXATTR

On Wed, Jun 26, 2019 at 05:11:33AM +0100, Al Viro wrote:
> On Tue, Jun 25, 2019 at 07:32:18PM -0700, Darrick J. Wong wrote:
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -373,10 +373,9 @@ static int check_xflags(unsigned int flags)
> >  static int btrfs_ioctl_fsgetxattr(struct file *file, void __user *arg)
> >  {
> >  	struct btrfs_inode *binode = BTRFS_I(file_inode(file));
> > -	struct fsxattr fa;
> > -
> > -	memset(&fa, 0, sizeof(fa));
> > -	fa.fsx_xflags = btrfs_inode_flags_to_xflags(binode->flags);
> > +	struct fsxattr fa = {
> > +		.fsx_xflags = btrfs_inode_flags_to_xflags(binode->flags),
> > +	};
> 
> Umm...  Sure, there's no padding, but still - you are going to copy that thing
> to userland...  How about
> 
> static inline void simple_fill_fsxattr(struct fsxattr *fa, unsigned xflags)
> {
> 	memset(fa, 0, sizeof(*fa));
> 	fa->fsx_xflags = xflags;
> }
> 
> and let the compiler optimize the crap out?

The v2 series used to do that, but Christoph complained that having a
helper for a two-line memset and initialization was silly[1] so now we
have this version.

I don't mind reinstating it as a static inline helper, but I'd like some
input from any of the btrfs developers (or you, Al) about which form is
preferred.

--D

[1] https://lkml.org/lkml/2019/6/25/533